The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.
The present invention relates generally to flight control actuators, and more particularly to an output shaft for a flight control actuation unit. Even more particularly, the present invention relates to a removable output shaft for a missile fin actuator in a missile control actuation unit.
Missile control fins are commonly positioned by actuators mounted within the missile body. Each control fin is usually coupled to a corresponding actuator by means of a cylindrical output shaft. The actuator, via the output shaft, exerts appropriate rotational torque and control on the fin so that missile control is achieved. In general, a high degree of torsional and bending stiffness is required of the actuator and its output shaft. Actuators may be electrically, pneumatically, or hydraulically powered, as is known.
A conventional actuator has a large outer bearing and significantly smaller inner bearing. The bearings are spaced apart at a distance D along the actuator output shaft. These bearings allow the shaft to rotate freely, and react to loads imposed by the output shaft as a result of aerodynamic loads on the fin. In conventional actuators, the distance D is the minimum bearing spacing compatible with maximum allowable bearing loads. These bearing loads are transferred to the actuator housing and reacted to by the missile body to achieve the desired missile control (e.g., attitude).
One shortcoming of such a conventional actuator design is that the bending stiffness needed to meet flutter requirements is limited by the small diameter of the inner bearing. As noted above, the diameter of the inner bearing is significantly smaller than the diameter of the outer bearing. The limited bending stiffness results, for example, in the onset of aerodynamic instability (flutter) at reduced airspeeds.
Another shortcoming of such a conventional actuator design is that the resultant reduced bending load capability reduces the maneuverability of the missile by reducing the loads which can be transferred to the bearings via the output shaft.
Yet another shortcoming of the conventional actuator design is the requirement for high manufacturing tolerances. Such high manufacturing tolerances are due, in part, to the unitary design of the actuation unit housing.
The conventional housing is usually a solid piece of aluminum in which several holes are bored out to receive each actuator unit and corresponding output shaft. A counter bore is required to produce a retaining shoulder to hold the smaller inner bearing. In the past, a counter bore of this nature could not be done automatically by a computer programmed boring machine. It had to be done manually, which drove up manufacturing costs. In addition, the unitary design made it difficult to service the output shafts in the field. This required that the entire missile be shipped back to the manufacture for servicing.
Further, the unitary design of the actuation unit housing makes it difficult to test actuators in the assembled units as required before delivery to a customer. Additionally, the unitary design made it difficult to test new missile fin attachment stub designs. In each of these sorts of tests, the missile actuation unit has to be taken substantially apart.
As a consequence, a need exists for improvement in actuation unit construction to reduce the cost associated with using, manufacturing, testing and servicing the output shaft of a missile control actuation unit and thereby eliminate costly corrective measures required to be taken as a result thereof.
The present invention provides an output shaft for a missile fin actuator in a missile control actuation unit designed to satisfy the aforementioned needs.
Accordingly, a shaft assembly for coupling a control fin to a missile includes outer and inner shaft portions which are detachably coupled together, allowing removal of one of the shaft portions while the other shaft portion remains in the missile. In an exemplary embodiment, the inner shaft portion, to which the control fin is coupled, is removable. The shaft assembly includes a pair of preload nuts which serve to adjust the position of the control fin relative to the skin of the missile, as well as to provide necessary bearing preload. The preload nuts may for example be engaged on opposite threaded ends of the outer shaft portion.
According to an aspect of the invention, a shaft assembly for coupling a control fin to a missile includes outer and inner shaft portions which are detachably coupled together. In an embodiment of the invention, the shaft portions are coupled together by means of a fastener.
According to another aspect of the invention, a shaft assembly for coupling a control fin to a missile includes a pair of bearings, the bearing closer to the center of the missile having approximately the same diameter as a bearing farther from the center of the missile.
According to still another aspect of the invention, a shaft assembly for coupling a control fin to a missile includes a shaft and a pair of preload nuts threadedly engaged with the shaft at respective opposite ends of the shaft. In an embodiment of the invention, the shaft includes an inner shaft portion coupled to an outer shaft portion, and the preload nuts are threadedly engaged with the outer shaft portion.
According to yet another aspect of the invention, a shaft assembly for coupling a control fin to a missile includes an inner shaft, an outer shaft detachably coupled to the inner shaft, and a pair of bearings coupled to the outer shaft for enabling rotation of the outer shaft relative to the missile.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.